It is known in the molding of synthetic resin materials from two reactive components, e.g. isocyanates and polyols in the production of polyurethanes or epoxide resins and amine hardeners in the production of epoxies, to utilize a mixing head connected to a mold cavity and having a cylindrical mixing chamber in which a control member, plunger, piston or ram is axially displaceable.
In one position of this control member, the end thereof is flush with the end of the mixing chamber and passages formed in the control member allow continuous recirculation of the two components from and to a supply vessel, e.g. via respective pumps, thereby preventing the development of static conditions which may result in blocking the inflow of the two reactive components.
In a second position, wherein the end of the control member is retracted from the end of the mixing chamber, inlets for the two components are unblocked and the two components are permitted to enter the mixing chamber for intimate mixing in the latter and discharge into the mold cavity.
Upon displacement of the control member from the second position into the first, residues of the mixture are forced out of the mixing chamber into the mold cavity and the aforementioned recirculation paths are reestablished.
Reference may be had to the following U.S. patents and the literature and art cited therein for a fuller description of such mixing heads: U.S. Pat. No. 3,706,515, U.S. Pat. No. 4,167,236, and U.S. Pat. No. 4,211,345.
It is also known, in connection with such mixing heads, to introduce into the mixing chamber a displacement or constricting body which, while occupying only a portion of the volume of the mixing chamber when the control member is retracted, serves to increase the pressure ahead of the inlets and mechanically obstruct partially the discharge of the mixture into the mold cavity. Reference may be had to German Pat. No. 20 65 841 in this connection.
The use of this displacement body markedly improves the intimacy of the mixing quality of the molded article.
Obviously, the operation of the displacement body, its insertion into the mixing chamber, must coincide with the shifting of the control member into its second or retracted position to prevent interference between this member and the displacement body. The displacement body is generally hydraulically actuated and in prior systems the control of its hydraulic displacement has been effected by a control signal generated by the movement of the control member.
In German patent document--Printed Application DE-AS No. 25 44 749, this control utilizes a position sensor responding to the position of the control member.
In German patent document--Open Application DE-OS No. 24 13 337, the control system for the hydraulic actuation of the control member is utilized to operate the displacement body as well via a separate but complex hydraulic network.
German patent document--Open Application--DE-OS No. 26 12 812 provides an electrohydraulic control for the displacement body which is triggered by a limit switch actuated by retraction of the control member.
In all of the aforementioned cases, the systems for operating the displacement body utilizes expensive and frequently unreliable components and may be sensitive to failure, e.g. of an electric current supply network or the like.